Fast acting servo mechanism



Oct. 17, 1961 1.1. DUF FY 31,004,390

FAST ACTING SERVO MECHANISM Original Filed Jan. 31, 1957 3 Sheets-Sheet1 III/IIIII/ INVENTOR.

James 1/. Duff] BY Q. "1 4.,

M KW fFq Arro/e/viys Oct. 17, 1961 J. J. DUFFY FAST ACTING SERVOMECHANISM 3 Sheets-Sheet 2 Original Filed Jan. 51, 1957 INVENTOR. JAWQSJ. Dun BY 844-.- Q i qfl 1961 J. J. DUFFY 3,004,390

' FAST ACTING SERVO MECHANISM Original Filed Jan. 51, 1957 s Sheets-Shet5 INVENTOR.

James J. Duffy BY s.L....- 4.. 7h'&

ATTOE/VEYS States 3,004,390 FAST ACTING SERVO MECHANISM James J. Duify,Detroit, Mich., assigno'r to Ford Motor Company, Dearborn, Mich., acorporation of Delaware Continuation of abandoned application SenNo.637,436,

Jan. 31, 1957. This application Aug. 11, 1959, Ser.

6 Claims. (Cl. 60-97) I My invention is particularly adapted to be usedfor energizing friction brake bands or clutches and it is capable ofproviding an initial, fast acting clutch or brake engaging motion fortaking up the free play in the clutch or brake operating linkage. Thisinitial motion is followed by a brake energizing force which increasesin successive stages thereby effecting a cushioned braking operation.The improved servo structure of my invention finds particular; utilityin automatic transmission mechanisms for motor vehicles wherein one ormore brakes or clutches are employed for regulating the relative motionof the transmission gear elements, a movable transmission clutch orbrake actuating member being mechanically connected to a fluid pressureoperated portion of the servo structure.

This application forms acontinuation of my pending application SerialNo. 637,436, filed January 31, 1957, now abandoned.

I am aware of other types of servo mechanisms of known construction foruse with friction brakes or the like which are capable of theabove-mentioned fast acting, slack take-up feature. However, the brakeactuating force applied by the servo mechanism increases very slowly inmagnitude during a time interval following initial brake engagement andthis is followed by a rather abrupt transition to a final brakeenergizing force of increased magnitude. If a brake or clutch mechanismemploys such a conventional servo mechanism and it is used to change theangular velocity of a rotating compo nent of an automatic transmissionmechanism, the rate of acceleration or deceleration of that componentwill be non-uniform and the shifting sequence from one gear ratio toanother may be lacking in smoothness.

One conventional transmission servo of this type is formed with bleedorifices in the relatively movable portions thereof for providing acontrolled communication between the fluid working chamber and the sumpregion of the transmission controls, the orifices providing a somewhatcushioned application of an associated brake member. ,However, theseorifices bleed oil during the initial stages of operation of the servoas well as during the final cushioning stage and the total time intervalrequired to energize this servo is accordingly increased to a valueexceeding the corresponding time interval of a servo of the samecapacity embodying my invention.

Also, the amount of oil required to operate the servo exceeds the actualfluid capacity of the servo working chamberand, if the system employs afluid pump as a pressure source, this necessitates an increase in thepumping requirements of the pump. Since transmission pumps are usuallydesigned to meet minimum requirements, the use of a servo mechanismhaving such a conventional fast acting feature would cause aconsiderable delay in the shift time interval. Also, the pressurebuildup in the working chamber of such a servo is substantially lessthan the pressure build-up which occurs in my improved servo mechanismduring the corresponding time interval al-' ate Patented Oct. 17, 1961though the latter is considerably shorter in the case of my improvedconstruction. 7

According to a principal feature of my invention I have provided animproved servo mechanism of the type above described wherein theoperating time interval is characterized by multiple stages, a firstoperating stage providing a fast acting, initial movement of theactuating member for eliminating the free play in the operating linkageand wherein the actuating force thereafter increases in subsequentstages until a maximum value is reached.

,7 It is a further object of my invention to provide a servo mechanismof the type above set forth which is simple in construction, and whichincludes acompound piston structure having relatively movable portionsdefining a first working chamber of minimum fluid capacity to effect aninitial take-up motion of the actuated memberv and a relatively largeworking chamber to provide a greater working area and to produce amaximum actuating force.

It is a further object of my invention to provide a servo mechanism asset forth above in the preceding objects wherein the transition from onestage to another is characterized by a maximum degree of smoothness.

It is another object of my invention to provide a servo mechanismwherein the time interval required to energize the same is substantiallyshorter than the corresponding time interval for conventional servomechanisms of comtion following the first operating stage during whichthe free play in the brake linkage is taken up; and

FIGURE 3 is a cross sectional view similar to FIG .URE 1 wherein theservo piston has assumed a final energized position.

Although I have illustrated the improved servo of my instant inventionwith a friction brake band of the type employed in automatictransmission of conventional construction, I contemplate that it mayalso be applied with success in other installations.

Referring first to FIGURE 1, numeral .10 is used to generally designatethe improved servo construction of my instant invention and it comprisesa main casing 12 which defines a working cylinder including a largediameter portion 14 and a relatively reduced diameter portion 16. Apiston assembly 18 is disposed within the casing 12 and it includes alarge diameter portion 20 acting in cooperation with the cylinderportion 14. The cylinder portion 20 is formed with a centrally situatedupper extension 22 and a lower skirt 24 which define a pilot cylinderfor slidably receiving a pilot piston 26. The piston 26 includes a head28 and a skirt 30 and it cooperates with its associated cylinder todefine a working chamber 32 of reduced volume. The head of the pistonportion 22 is formed with a central aperture 34 within which ispositioned a movable valve element 36, the latter comprising a headforming a tapered valve seat 38 and a stem 40 which extends through theopening 34. The stem 40 is formed with an annular shoulder which servesas a seat-for a conical spring 42, the other end of spring42 seating onthe base of a relatively large diameter extension 44 of the opening 34.

As illustrated in FIGURE 1, the stem 40 of the valve 36 contacts theupper end wall of the cylinder portion 3 16 and compresses the spring 42to lift the valve seat 38 away from the opening 34 to providecommunication between the working fluid chamber 32 and the workingchamber defined by the outer working cylinder portions 14 and '16 abovethe piston '20. The piston 20 is urged in an upward direction, as viewedin FIGURE 1, by conical compression spring 46 which is seated on aclosure plate 48 located in the lower end of the cylinder portion .14.The closure plate 48 is maintained in place by a conventional snap ring50. A precalibrated orifice 51 is formed in the upper end of the pistonextension 22 to provide a controlled degree of communication between thefluid pressure chamber 32 and the working chamber on the upper side ofpiston 20. The function of orifice 51 will subsequently be described inthe state' ment of the operation of the invention.

' The base 28 of pilot piston 26 is rounded to provide a convex bearingsurface 52 which is adapted to engage a mating concave surface formed onthe end of a rocker arm 54, the latter extending through an aperturelocated in the closure plate 48. The end of the rocker arm 54 isrotatably journaled on a pin 56 which in turn is supported by a boss 58which, by preference, is formed integrally with the casing 12 of theservo structure. The boss 58 is bipartite in form and it defines a spacesituated between two walls within which the end of the rocker arm 54extends, the pin 56 bridging the space. The proximate end of the rockerarm 54 may be connected to an extension 60 of an end section 64 of afriction brake band indicated at 66. The effective length of the link 62may be varied by means of a suitable threaded adjustment 68. The bandend section 64 is formed with a notch 70 which cooperates with the link62 as indicated. V

The band 66 encircles a brake drum 72 which may form a portion of a geararrangement of a multiple speed transmission or the like. The other endof the brake band 66 is formed with a shoulder portion 74 which isadapted to be anchored by a contacting portion 76 of the cylinder casing12, the casing 12 thereby being effective to absorb the brake reactiontorque. The casing 12 may be permanently anchored to the transmissionhousing to maintain the same in a relatively stationary position.

Fluid pressure may be admitted to the working chamber on the upper sideof the piston 20 through passages 78, 80 and 82. These passages may forma portion of a complete transmission control circuit which is notillustrated.

During operation, the servo structure of my invention assumes thecondition shown in FIGURE 1 when the brake drum 72 rotates freely aboutits axis. When b a ing of the drum 72 becomes necessary during a shiftvsequence, fluid pressure is introduced into the working cylinder 12above the piston 20 through the passages 78, 80 and .82. Since the valve36 assumes a fully opened position as indicated in FIGURE 1, fluidpressure will immediately be admitted to the working chamber 32 definedby the pilot piston 26. Since the fluid capacity of the working chamber32 is relatively small, the Pilot piston .26 will move qui kly t anextende P ition as indicated in. FIGURE 2 thereby t king p th free playin the brake actu ting l nk ge o c use he brak transmitted to the rockerarm 54 through the medium of the fluid in the chamber 32 and the spring46 will begin to compress as the fluid pressure above the piston 20continues to increase. The preload of the spring 46 is sufficient tomaintain the piston 20 in a fully upward position until the pilot piston28 has assumed its fully extended position.

If the fluid pressure in the working chamber above the main piston 20continues to increase, the piston 20 will continue to move until itassumes the position shown in FIGURE 3 at which time the pilot piston 26bottoms against the piston extension 22. Since an increase in fluidpressure will no longer be associated with a change in the volume of theeifective working chamber after the main piston 20'assumes the positionshown in FIG- URE 3, the fluid pressure will immediately increase to itsmaximum value as determined by a main pressure regulator valve locatedin the control circuit.

It is apparent from the inspection of FIGURES 2 and 3 that the capacityof the working chamber 32 decreases as the main piston 20 moves from theposition shown in FIGURE 2 to that shown in FIGURE 3. The precalibratedorifice 51 is adapted to accommodate the transfer of fluid from theinterior of the working chamber 32 to the main working chamber above themain piston 20 during the downward motion of the latter. This fluidmetering action provides a cushioned brake energizing operation. It isemphasized that the amount of fluid which is necessary to completelyenergize the servo mechanism is equal in magnitude to the fluid capacityof the working chamber above the main piston 20 and, except for normalleakage, all of the fluid supplied to the servo mechanism is effectivelyutilized. Also, after the initial hand 66 to come in con act with thedrum 72. Thi

action takes plac r p dly y rea on of th fa h onl a relatively smallvolume of oil must b intr uced into the compound piston structure toeffec th r quired take-up motion- Following this initial movement of thepilot piston, the fluid pressure in the chamber above the piston 20 willbeg n t in rease and the p t n .20 will begin to move in a own arddirectio as viewed in FIGURES 1 through 3. After a slight initialmovement of the main piston 20, the valve 36 assumes a closed positionunder the influence of the spring 42 thereby trapping the fluid in theworking chamber 2. The pressure force of the fluid above the main piston20 will be slack take-up motion of the pilot piston has been completedand after the valve 36 assumes a closed position, the pressure in theworking chamber 3-2 increases to a valve exceeding the value of thepressure build-up in the chamber above the main piston 20. Since thefluid contained in the working chamber 32 is caused to bleed through theorifice 51 by reason of the pressure diiferential thereacross, thispressure build-up in the main working chamber will be accompanied byv acorresponding pressure build up in the working chamber 32 in order thatthe restricted fluid flow through the orifice 51 might be continued; butthe actual static pressure in working chamber 32 will be greater thanthe corresponding static pressure in the main working chamber duringthis operating stage. Since the effective force on the end of the crank54 is a direct function of the pressure existing in the working chamber3-2 during the downward motion of the piston 20, the eifective forcetransmitted to the brake band 66 will be correspondingly increasedduring relative telescopic movement of the pistons. Further, if anorifice between the Working chamber 32 and th ump for the transmi sioncontrols would be su it e f r the orifice 51 in the disclosedembodiment, compensation would be necessary for the fluid which would beexhausted therethrough. Since the time interval required to complete theoperating sequence is dependent upon the total fluid necessary tooperate the compound pistons, and since part of the fluid supplied tothe pistons is thus lost, the, total time interval would beproportionately increased.

After the pilot piston bottoms against the main piston 20, the totaleffective pressure in the working chamber 32 in applicants structurewill have reached a precalibrated limiting value which approaches themaximum line pressure made available by the pressure source. Thepressure increase which takes place the instant the pilot piston bottomsagainst the main piston 20' may be readily predetermined for any givendesign.

It is thus apparent from the foregoing description that the servostructure of my instant invention is effective to substantially reducethe operati g ime While imu tan u ly P vid ng or a pressur increase insuc e s ve stages during the shortened operating time interval. Both ofthese features are desirable in producing a smooth transmission shiftsequence.

What I claim and desire to secure by United States Letters Patent is:

1. A fluid pressure operated servo mechanism for actuating a movablemember comprising a cylinder, a compound piston movably disposed withinsaid cylinder, said piston comprising a main piston portion defining inpart a main fluid Working chamber within said cylinder and a pilotpiston portion slidably disposed within said main piston portion intelescopic relationship therewith, said pilot piston portion and saidmain piston portion defining another fluid working chamber of lesservolume than the main fluid working chamber, a check valve formed in saidmain piston portion providing a one-way transfer of fluid from said mainWorking chamber to said other working chamber, said pilot piston portionbeing adapted to actuate said movable member, and a precalibratedorifice in said main piston portion positioned to provide continuous,restricted fluid flow between said working chambers during relativetelescopic movement of said piston portions whereby a cushionedactuating force is applied to said movable member during an initialoperatting interval and wherein an intermediate actuating force build-upoccurs during a subsequent operating interval upon relative movement ofsaid piston portions, said force build-up being followed by a finalsustained operating force upon positive engagement of said pistonportions.

2. A fluid pressure operated servo mechanism for actuating a movable,work performing member and comprising a cylinder, a compound pistonincluding a main piston portion telescopically received within saidcylinder and defining therewith a first working chamber, said compoundpiston further including a pilot piston portion movable relative to saidmain piston portion in telescopic relationship therewith, said pistonportions defining a second working chamber having a lesser volume thanthe first working chamber, a check valve formed in said main pistonportion providing one-way fluid communication between said workingchambers, said check valveineluding a portion positioned to be engagedwith a wall of said cylinder to open the same when said main pistonportion assumes a deenergized, limiting position, and a flow restrictingorifice in said main piston portion positioned to provide continuous,restricted fluid flow between said working chambers during relativetelescopic movement of said pilot piston portion and said main pis tonportion, said pilot piston portion being adapted to actuate saidmovable, work performing member, the operation of said servo mechanismbeing characterized by multiple operating stages whereby an initialactuating force of reduced magnitude is produced by the fluid pressurein said second working chamber and whereby an intermediate pressurebuild-up subsequently occurs in said first working chamber upon movementof said main piston portion relative to said pilot piston portion, thefluid displaced from said second working chamber during relativemovement of said piston portions beingmetered ghrough said orificedirectly into said first Working cham- 3. A fluid pressure operatedservo mechanism for actuating a work performing member and comprising acasing, said casing defining a main fluid working cylinder, a compoundpiston disposed within said main cylinder, said compound pistonincluding a main piston portion telescopically related with respect tosaid cylinder and defining a main fluid working chamber, a pilotcylinder formed in said compound piston in coaxial relationship withrespect to said main cylinder, said compound piston further including apilot piston portion movably situated in said pilot cylinder anddefining therewith a secondary fluid working chamber, a one-way checkvalve formed in said main piston portion and adapted to accommodate thetransfer of pressurized fluid from said main working chamber to saidsecondary working chamber, a flow restricting orifice located in saidmain piston portion and positioned to provide continuous, restrictedfluid flow between said working chambers during relative telescopicmovement of said piston portions, and spring means for urging said mainpiston portion toward a de-energized, limiting position, said checkvalve being engageable with a relatively stationary part of saidcylinder to cause the same to open when said main piston portion assumessaid limiting position, said pilot piston portion being adapted toactuate said work performing member, the operation of said servomechanism being characterized by multiple operating stages whereby aninitial actuating force of reduced magnitude is produced by the fluidpressure in said secondary working chamber and whereby an intermediatepressure build-up subsequently occurs in said main working chamber uponmovement of said main piston portion relative to said pilot pistonportion, the fluid displaced from said working chamber during relativemovement of said piston portions being metered through said orificedirectly into said main working chamber.

4. The combination as set forth in claim 3 wherein said main pistonportion is substantially T-shaped in cross section with respect to aplane containing the axis of said main cylinder, the radially inwardparts of said main piston portion being elongated in the direction ofthe axis of said main cylinder to define said pilot cylinder.

5. The combination as set forth in claim 4 wherein one end of said mainfluid working chamber is similar in shape to the cooperating parts ofsaid main piston portion thereby defining a minimum working volume for agiven working area.

6. In a multiple stage fluid pressure operated mechanism for actuating amovable member, a cylinder, a compound piston movably disposed withinsaid cylinder, said compound piston comprising a main piston portiondefining in part a main fluid working chamber within said cylinder and apilot piston portion adapted for relative telescopic movement withrespect to said main piston portion, said pilot piston portion and saidmain piston portion defining a secondary working chamber of lesservolume than said main fluid working chamber, a check valve means in saidcompound piston for providing a oneway transfer of fluid from said mainworking chamber to said secondary working chamber, and a precalibratedorifice formed in one of said piston portions adapted to providerestricted fluid flow between said working chambers during relativetelescopic movement of said main piston portion and said pilot pistonportion, said pilot piston portion being adapted to actuate said movablemember, a first operating stage of said servo mechanism beingcharacterized by an initial actuating force of reduced magnitudeproduced by the fluid pressure in said secondary working chamber and asubsequent operating stage of said servo mechanism being characterizedby an intermediate pressure build-up in said main working chamber uponmovement of said main piston portion relative to said pilot pistonportion, the fluid displaced from said secondary working chamber duringrelative movement of said pilot piston portion with respect to said mainpiston portion being metered through said orifice directly into saidmain working chamber.

References Cited in the file of this patent UNITED STATES PATENTS

